1. Field of the Invention
This invention relates to a small, precision thermometer for measuring cryogenic temperatures.
2. Description of the Prior Art
The vapor pressure which is produced when a gas is liquefied is always stable and, therefore, is widely used as the standard for the graduation of temperature. A thermometer which uses the vapor pressure of a liquefied gas as the standard graduation generally requires a bulky pressure measuring system which introduces an element of error into the measurement since it is located in an atmosphere at room temperature. The disadvantage of the conventional thermometer makes it desirable to provide a small, precision vapor-pressure thermometer for the measurement of cryogenic temperatures.
To meet this need, a vapor-pressure thermometer having a gas specimen sealed in a container at room temperature under a prescribed pressure in conjunction with a pressure sensitive element has been devised. When the sealed gas once liquefied at cryogenic temperatures returns to room temperature, its pressure increases to several tens of times the condensing pressure. The pressure sensitive element used in the container is required to be resistant to this pressure. Therefore, the pressure sensitive element is inevitably designed to provide this resistance at a sacrifice of sensitivity.